Memory CD8 T cells can provide exceptionally long-term protective immunity against reinfection due to their enhanced longevity, proliferative capacity and functional potential. During a viral infection, the activated CD8 T cells clonally expand and acquire cytotoxic function to eliminate infection. However, after viral clearance, antiviral effector CD8 T cells rest down and stop dividing and synthesizing antiviral proteins. A portion of the effector cells differentiate and mature into protective memory CD8 T cells that are long-lived, have a high proliferative potential and can replenish the memory pool by homeostatic proliferation. Recent work suggests that this effector`memory (E`M) transition requires a metabolic switch from an anabolic, rapidly dividing state that relies primarily on glucose and glycolysis (driven by the PI3K/AKT/mTOR pathway) to a catabolic, quiescent state that relies on fatty acids/ fatty acid oxidation (FAO; driven by Foxo/AMPK activity). However, this model requires substantial investigation because very little is known about memory CD8 T cell metabolic states, which signals regulate this process and how this affects their lifespan and function. Perhaps more importantly, we need to better understand how changes in CD8 T cell metabolism are coordinated with T cell differentiation during immune responses. We hypothesize that STAT3 lies at the cross roads of these two processes because it is both a transcription factor that controls memory T cell differentiation and a mitochondrial protein that modulates mitochondrial respiration. Our preliminary data show that IL-21/IL-10/STAT3 signaling is essential for memory CD8+ T cell development during an acute viral infection. In absence of either IL-21 and IL-10 or STAT3, virus-specific CD8+ T cells retain terminal effector (TE) differentiation states and fail to mature into functional memory T cells that protect against reinfection and contain self-renewing TCM cells. Thus, STAT3 activity drives the E`M transition and is necessary for functional, protective memory CD8 T cells to form after acute LCMV infection. Additionally, we have data suggesting that STAT3 controls both the expression of key transcription factors involved in memory CD8 T cell differentiation and the activity of the PI3K/AKT/mTOR and AMPK pathways that regulate memory T cell metabolism, differentiation, function and survival. These data suggest STAT3 integrates various signals and coordinately regulates both T cell differentiation and metabolism. Our primary aim in this proposal is to determine if STAT3 regulates an anabolic ` catabolic switch during memory CD8 T cell differentiation. To study this, we will determine whether (1) PI3K/AKT/mTOR activity is abnormally increased and, reciprocally, if AMPK activity is decreased in Stat3-/- virus-specific CD8 T cells and (2) if STAT3 controls glycolysis, oxidative respiration and fatty acid oxidation in virus-specific CD8 T cells. Lastly, we will (3) compare the roles of nuclear and mitochondrial STAT3 in memory CD8 T cell development, metabolism and gene expression. ) )